In many composite materials, and particularly in composite materials that include rubber and polymeric fibers, adhesion between the polymeric fibers and the rubber is a significant factor in the overall performance of such composite materials, and numerous approaches are known in the art to improve adhesion between a polymeric fiber and a rubber composition.
In one approach, the surface of a polymeric fiber is physically modified with an electron beam to create particular chemically reactive groups that are then involved in forming covalent bonds between the polymeric fiber and the rubber as described in U.S. Pat. No. 4,794,041 to Gillberg-LaForce et al. Treating polyethylene terephthalate (PET) materials with an e-beam advantageously generates reactive groups (e.g., carboxylate or hydroxyl groups) on the PET surface, which may react with epoxy or isocyanate groups in the adhesive coating on the polymeric fiber to improve bonding between the PET material and a rubber. However, e-beam treatment tends to increase production cost. Moreover, Gillberg-LaForce's approach typically requires installation of relatively large and energy consuming equipment.
In another approach, the surface is chemically modified with a solvent to improve adhesion of a rubber composition of the fiber as described, for example, in U.S. Pat. No. 3,644,136 to McCullough et al or U.S. Pat. No. 3,902,950 to Tung et al. Surface treatment of a polymeric fiber is relatively simple, and may even be performed in some cases without specialized equipment. However, the use of halo-organic solvents in such treatments frequently poses environmental and/or health concerns. Moreover, surface treatment with halo-organic solvents typically still necessitates in many cases the use of resorcinol-formaldehyde-latex (RFL) to improve rubber adhesion.
To circumvent at least some of the problems associated with surface treatments, polymeric fibers may be covered with an additional layer. Such layers may be applied in the form of a spin finish, overfinish, or by dipping the polymeric fiber into one or more adhesives. There are numerous examples known in the art, and exemplary methods and compositions for spin-finish coating are described in U.S. Pat. No. 4,467,064 to Kudo et al. or U.S. Pat. No. 4,477,497 to Kudo et al Exemplary methods and compositions for over-finish coating are described in JP 8,246,353 to Kitahara et al., and exemplary methods and compositions for dip coating are described in U.S. Pat. No. 3,429,354 to Malcom or U.S. Pat. No. 3,903,332 to Kelly et al. Although providing additional layers onto the surface of polymeric fibers frequently improves the adhesion of rubber to the fibers, the additional layers generally still need an RFL adhesive coating, which is environmentally problematic and may pose significant heath threats to workers exposed to resorcinol and formaldehyde.
In still other approaches, RFL is replaced with an acrylic resin with a molecular weight of between about 100,000 and 1,000,000 that is mixed with resorcinol-formaldehyde free latex as described by Solomon et al. in U.S. Pat. No. 4,472,463. While the use of Solomon's resins allows omission of RFL and tends to improve adhesion of latex to a substrate, the use of such resins typically requires a two-step dip process (e.g., a polyepoxide dip and an acrylic-rubber latex mixture). Moreover, Solomon's resins lack crosslinking functionalities to the latex, thereby potentially reducing optimal adhesion.
Alternatively, maleinized polybutadiene may be employed to eliminate the use of RFL as described in a paper presented at the 155th Rubber Division of the ACS, Chicago, Ill., Apr. 13–16, 1999 (“Maleinized Polybutadiene Latex for Fiber to Rubber” by A. S. Estrin and R. W. Nalepa). Although such compositions may advantageously replace RFL in some instances, maleinized polybutadiene has relatively poor water solubility and generally requires the use of sodium, potassium or ammonium hydroxides for formation of lattices and solutions in water. Moreover, adhesion of such compositions to fibers is at least in some cases less than desirable.
Although various methods and compositions for improving adhesion between a polymeric fiber and rubber are known in the art, all or almost all of them suffer from one or more disadvantages. Thus, there is still a need to provide compositions and methods for improved adhesion promoters.